Solder paste for bonding micro components

ABSTRACT

A solder paste for soldering micro components which suppress electro-migration is provided. The solder paste comprising: a powdered Pb-free soldering alloy comprising Cu of 0.1˜3.0 wt. % and In of 0.5˜8.0 wt. % simultaneously in addition to Sn; and a pasty or liquid flux; the powdered Pb-free soldering alloy and the pasty or liquid flux being mixed; whereby effectively suppressing electro-migration occurring at a solder bonding portion. It is possible to add Ag, Sb, Ni, Co, Fe, Mn and Cr as a strength improvement element, Bi and Zn as a melting point drop element and P, Ga and Ge as an anti-oxidant element to the powdered Pb-free soldering alloy.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a solder paste for bonding micro components, which suppresses electro migration.

2. Background Art

So far a soldering alloy of Sn—Ag—Cu or Sn—Cu has been widely used which was proposed in Japanese patent No. 3027441 as so called Pb-free soldering alloy that is almost without Pb.

When soldering a micro component to a print circuit a reflow soldering method which uses a pasty soldering paste that is prepared by mixing a powdered soldering alloy and a pasty or liquid flux is widely adopted.

Electro-migration is a phenomenon where at a metal wiring portion or a solder bonding portion, diffusion of metal atoms occurs by an interaction between metal atoms and electrons running in the wired portion and metal atoms migrate toward a direction of an electric current. It is a well known fact that electro-migration occurs notably when a current density is high.

In the field of semiconductors refinement of wiring has been pursued due to rapid expansion of semiconductor demands. This caused occurrence of electro-migration through high current density in a metal wiring to make disconnection of the metal wiring, which became a major issue.

A solution for a disconnection of Aluminum wiring was proposed in a Japanese patent No. 3021996 which teaches a use of Aluminum wiring with uniformly added Scandium, whereby preventing an occurrence of electro-migration.

Meanwhile a possibility of occurrence of electro-migration at a solder bonding portion has been regarded as small as its area is larger than that of a metal wiring used in semiconductor field. However, the area of a solder bonding portion has become considerably smaller together with refinement of a circuit board and components on it and emergence of reflow soldering method as a main stream of soldering.

For example, a chip capacitor and a chip resistor are soldered through reflow soldering method using a solder paste. Recently sizes of a chip capacitors and a chip resistor are refined remarkably and models such as ‘0603’ with length of 0.6 mm and width of 0.3 mm, and ‘0402’ with length of 0.4 mm and width of 0.2 mm are widely used. Further, a model of ‘0201’ with length of 0.2 mm and width of 0.1 mm is now under development.

Under these circumstances where increased current density together with decreased area of a solder bonding portion is anticipated an occurrence of electro-migration at a solder bonding portion is concerned. When electro-migration occurs at a solder bonding portion there is a possibility that the soldering alloy would have a crack and an exfoliation and as a result electrical disconnection would occur. The disconnection at the solder bonding portion would cause a malfunction of an electronic device and may result in a fatal accident.

DISCLOSURE OF INVENTION

Recently the solder bonding portion has been refined along with the refinement of circuit boards and as a result an occurrence of electro-migration at a solder bonding portion due to high current density is concerned.

Therefore an object of present invention is to provide a Pb-free solder paste which suppresses an occurrence of electro-migration at a solder bonding portion.

In order to fulfill the above object, present invention provides a solder paste for bonding micro components comprising:

a powdered Pb-free soldering alloy comprising Cu of 0.1˜3.0 wt. %, In of 0.5˜8.0 wt. %, Sn and unavoidable impurities of the rest; and

a pasty or liquid flux;

the powdered Pb-free soldering alloy and the pasty or liquid flux being mixed;

whereby effectively suppressing electro-migration occurring at a solder bonding portion.

The soldering alloy used in the present invention comprises Cu of 0.1 to 3.0 wt. %, In of 0.5 to 8.0 wt. % and Sn and unavoidable impurities of the balance. When either Cu or In alone is added to the alloy, the effectiveness of suppressing the occurrence of electro-migration at the solder bonding portion is low. However when both of Cu and In are added at a certain wt. %, the occurrence of electro-migration at the solder bonding portion is effectively suppressed.

The mechanism of suppressing the occurrence of electro-migration has not been elucidated yet. However it is known that in a soldering alloy Cu makes a Sn—Cu (Cu6Sn5) inter-metallic compound and In forms a SnIn solid solution wherein Sn dissolves.

Electro-migration is a phenomenon where a diffusion of metal atoms occurs by an interaction between metal atoms and electrons running in a wiring portion at a solder bonding portion and metal atoms migrate toward a direction of electric current. Therefore it is reasonable to consider that especially when the current density is high, Sn—Cu inter-metallic compounds and SnIn solid solution with which the soldering alloy is dotted, may suppress the movement of metal atoms with considerable effectiveness.

When both of Cu and In are added to the soldering alloy and added amount of Cu is equal to or less than 0.1 wt. % or added amount of In is equal to or less than 0.5 wt. % the effectiveness of suppressing the occurrence of electro-migration is low. Also when both of Cu and In are added to the soldering alloy and the amount of Cu is more than 3.0 wt. %, a liquidus temperature of the soldering alloy may go higher and liquidity of the soldering alloy may be reduced and as a result its solderability may be deteriorated. Therefore this composition of Cu with more than 3.0 wt. % is inadequate for a soldering alloy. Also when both of Cu and In are added to the soldering alloy and the amount of In is more than 8.0 wt. %, the liquidus temperature of the soldering alloy may be lowered considerably and the soldering alloy at a solder bonding portion may partly melt in case it is exposed to a high temperature. In addition, as In is high priced recently it not adequate to include In excessively.

A soldering alloy comprising Cu of 0.1 to 3.0 wt. % and In of 0.5 to 8.0 wt. % in addition to Sn effectively prevents an occurrence of electro-migration at a solder bonding portion. However its strength is not enough when considerably high strength is required to the solder bonding portion. In such cases it is possible to add either one or more elements of Ag, Sb, Ni, Co, Fe, Mn, Cr as an element for improving mechanical characteristics and strength of the solder bonding portion. These metallic elements have an effect of improving mechanical characteristics through forming a solid solution or an inter-metallic compound with the main component Sn. However an excessive amount of these may raise the liquidus temperature. Therefore it is preferable that the amount of Ag and/or Sb is not more than 4.0 wt. % and the total amount of Ni, Co, Fe, Mn and Cr is not more than 0.3 wt. % respectively.

When it is necessary to lower the reflow soldering temperature, for example to prevent thermal degradation of electronic components mounted on a printed circuit board, it is possible to add either one or both of Bi, Zn as a melting point drop element. However when the amount of Bi is excessive brittleness of the soldering alloy may increase. Also when the amount of Zn is excessive the soldering alloy may be oxidized at the time of melting and as a result its solderability is deteriorated. Therefore it is preferable that the amount of Bi and/or Zn is not more than 3.0 wt. %.

Further, it is possible to add either one or more of P, Ga and Ge as an anti-oxidant element to prevent oxidization of the soldering alloy at the time of reflow soldering. However when the amount of these is excessive the brittleness of the soldering alloy may increase and its solderability may be deteriorated. Therefore it is preferable that the total amount of P, Ga and Ge is not more than 0.3 wt. %.

MODE FOR CARRYING OUT THE INVENTION

The present invention will be described below based on embodiments.

Pb-free soldering alloy powder (average grain size of 20˜30 micrometer) with compositions described in the Table 1 and Table 2 which include both Cu and In in addition to Sn is prepared. Then the soldering alloy powder of 90 wt. % and a pasty flux of 10 wt. % are mixed to form a solder paste.

The solder paste was print-painted on a circuit board for mounting chip components using a mask for painting. Then a test chip component (a Nickel plated Cupper plate with 3.2 mm in length, 1.6 mm in width, 0.6 mm in thickness) was mounted on the painted portion. Then the circuit board was heated in a reflow oven and the solder paste was melt to form soldering between the test chip and the circuit board. After soldering was finished, the circuit board was left in an environment of 120 degree C. and a current of 10 A was imposed on the solder bonding portion. After 100 hours a cross section of the solder bonding portion was examined to detect occurrence of electro-migration. The test result was as shown in Table 3.

TABLE 1 Composition of powdered Pb free solder alloy Weight % Sn Cu In Ag Sb Ni Co Mn Fe Cr Example 1 the rest 0.1 0.5 Example 2 the rest 0.7 0.5 Example 3 the rest 0.7 5.0 Example 4 the rest 3.0 5.0 Example 5 the rest 0.7 8.0 Example 6 the rest 0.7 0.5 0.3 Example 7 the rest 3.0 8.0 1.0 Example 8 the rest 0.1 0.5 1.0 Example 9 the rest 0.5 3.0 3.0 Example 10 the rest 0.7 3.0 4.0 Example 11 the rest 0.1 3.0 0.5 Example 12 the rest 3.0 8.0 4.0 Example 13 the rest 0.7 0.5 1.0 0.5 Example 14 the rest 0.1 8.0 3.0 3.0 Example 15 the rest 0.7 0.5 0.1 Example 16 the rest 3.0 0.5 0.2 Example 17 the rest 0.7 8.0 0.1 Example 18 the rest 0.1 0.5 0.1 Example 19 the rest 3.0 8.0 0.2 Example 20 the rest 0.7 3.0 0.1 Example 21 the rest 0.1 3.0 0.1 Example 22 the rest 3.0 8.0 0.2 Example 23 the rest 0.7 0.5 0.05 Example 24 the rest 0.1 0.5 0.05 Example 25 the rest 0.7 3.0 0.05 Example 26 the rest 3.0 8.0 0.2 Example 27 the rest 0.1 3.0 0.05 Example 28 the rest 3.0 8.0 0.2 Example 29 the rest 0.7 0.5 0.1 0.1 Example 30 the rest 0.1 3.0 0.1 0.1 0.1 Example 31 the rest 3.0 8.0 0.1 0.2 Example 32 the rest 0.7 3.0 0.1 0.1 0.05 Example 33 the rest 3.0 3.0 0.1 0.1 0.05 0.05 Example 34 the rest 0.7 0.5 0.1 0.05 Example 35 the rest 0.7 0.5 0.2 0.05 0.05 Example 36 the rest 0.7 0.5 0.3 0.1 Example 37 the rest 3.0 0.5 1.0 0.2 Example 38 the rest 0.5 8.0 3.0 0.1 Example 39 the rest 0.1 0.5 0.3 0.1 Example 40 the rest 3.0 8.0 1.0 0.2 Example 41 the rest 0.5 3.0 3.0 0.1 Example 42 the rest 0.1 3.0 3.0 0.1 Example 43 the rest 3.0 8.0 3.0 0.2 Example 44 the rest 0.7 0.5 0.3 0.05 Example 45 the rest 0.1 0.5 1.0 0.05 Example 46 the rest 0.5 3.0 3.0 0.05 Example 47 the rest 3.0 8.0 4.0 0.2 Example 48 the rest 0.1 3.0 3.0 0.05 Example 49 the rest 3.0 8.0 4.0 0.2 Example 50 the rest 0.7 0.5 0.3 0.1 0.1 Example 51 the rest 0.5 3.0 3.0 0.1 0.1 0.1 Example 52 the rest 0.7 8.0 4.0 0.1 0.2 Example 53 the rest 0.7 3.0 4.0 0.1 0.1 0.05 Example 54 the rest 0.7 3.0 4.0 0.5 0.1 0.1 0.05 0.05 Example 55 the rest 0.5 0.5 3.0 0.1 0.05 Example 56 the rest 0.5 0.5 3.0 0.2 0.05 0.05 Comparative the rest 0.7 Example 1 Comparative the rest 3.0 Example 2 Comparative the rest 0.5 Example 3 Comparative the rest 8.0 Example 4 Comparative the rest 0.7 0.3 Example 5 Comparative the rest 0.7 1.0 Example 6 Comparative the rest 0.5 3.0 Example 7

TABLE 2 Composition of powdered Pb free solder alloy Weight % Sn Cu In Ag Sb Bi Zn P Ga Ge Exmaple 57 the rest 0.7 0.5 0.5 Exmaple 58 the rest 0.1 3.0 0.5 Exmaple 59 the rest 0.7 8.0 0.5 Exmaple 60 the rest 0.7 0.5 1.0 Exmaple 61 the rest 0.7 3.0 3.0 Exmaple 62 the rest 3.0 3.0 3.0 Exmaple 63 the rest 0.1 3.0 1.0 Exmaple 64 the rest 3.0 8.0 3.0 Exmaple 65 the rest 0.7 0.5 1.0 1.0 Exmaple 66 the rest 3.0 8.0 0.1 Exmaple 67 the rest 0.1 0.5 0.1 Exmaple 68 the rest 0.7 0.5 0.1 Exmaple 69 the rest 0.7 3.0 0.1 Exmaple 70 the rest 0.7 3.0 0.1 0.1 Exmaple 71 the rest 3.0 0.5 0.1 0.1 0.1 Exmaple 72 the rest 0.7 0.5 3.0 0.1 Exmaple 73 the rest 0.7 3.0 3.0 0.1 Exmaple 74 the rest 0.7 3.0 3.0 0.1 0.1 Exmaple 75 the rest 0.7 0.5 0.3 0.5 Exmaple 76 the rest 0.1 3.0 1.0 0.5 Exmaple 77 the rest 0.7 8.0 3.0 0.5 Exmaple 78 the rest 0.5 0.5 3.0 1.0 Exmaple 79 the rest 0.5 3.0 3.0 3.0 Exmaple 80 the rest 0.7 3.0 4.0 3.0 Exmaple 81 the rest 0.5 3.0 3.0 1.0 Exmaple 82 the rest 3.0 8.0 0.3 3.0 Exmaple 83 the rest 0.5 0.5 3.0 0.5 1.0 1.0 Exmaple 84 the rest 3.0 8.0 0.3 0.1 Exmaple 85 the rest 0.5 0.5 3.0 0.1 Exmaple 86 the rest 0.7 0.5 0.3 0.1 Exmaple 87 the rest 0.5 3.0 3.0 0.1 Exmaple 88 the rest 0.1 3.0 3.0 0.1 0.1 Exmaple 89 the rest 3.0 0.5 3.0 0.1 0.1 0.1 Exmaple 90 the rest 0.7 0.5 0.3 3.0 0.1 Exmaple 91 the rest 0.5 3.0 3.0 3.0 0.1 Exmaple 92 the rest 0.5 3.0 3.0 0.5 3.0 0.1 0.1 Comparative the rest 58.0 Example 8 Comparative the rest 9.0 Example 9

TABLE 3 Test Results of Electro-migration Occurence of electro-migration at the solder bonding portion Example 1 No occurrence (intermetallic com- 2.6 μm) pound layer at positive pole: Example 2 No occurrence (intermetallic com- 2.5 μm) pound layer at positive pole: Example 3 No occurrence (intermetallic com- 2.5 μm) pound layer at positive pole: Example 4 No occurrence (intermetallic com- 2.3 μm) pound layer at positive pole: Example 5 No occurrence (intermetallic com- 2.7 μm) pound layer at positive pole: Example 6 No occurrence (intermetallic com- 3.0 μm) pound layer at positive pole: Example 7 No occurrence (intermetallic com- 2.5 μm) pound layer at positive pole: Example 8 No occurrence (intermetallic com- 2.4 μm) pound layer at positive pole: Example 9 No occurrence (intermetallic com- 2.5 μm) pound layer at positive pole: Example 10 No occurrence (intermetallic com- 3.1 μm) pound layer at positive pole: Example 11 No occurrence (intermetallic com- 2.2 μm) pound layer at positive pole: Example 12 No occurrence (intermetallic com- 2.4 μm) pound layer at positive pole: Example 13 No occurrence (intermetallic com- 2.2 μm) pound layer at positive pole: Example 14 No occurrence (intermetallic com- 2.7 μm) pound layer at positive pole: Example 15 No occurrence (intermetallic com- 2.2 μm) pound layer at positive pole: Example 16 No occurrence (intermetallic com- 2.6 μm) pound layer at positive pole: Example 17 No occurrence (intermetallic com- 2.5 μm) pound layer at positive pole: Example 18 No occurrence (intermetallic com- 2.5 μm) pound layer at positive pole: Example 19 No occurrence (intermetallic com- 3.2 μm) pound layer at positive pole: Example 20 No occurrence (intermetallic com- 2.8 μm) pound layer at positive pole: Example 21 No occurrence (intermetallic com- 3.3 μm) pound layer at positive pole: Example 22 No occurrence (intermetallic com- 3.0 μm) pound layer at positive pole: Example 23 No occurrence (intermetallic com- 2.6 μm) pound layer at positive pole: Example 24 No occurrence (intermetallic com- 2.3 μm) pound layer at positive pole: Example 25 No occurrence (intermetallic com- 2.4 μm) pound layer at positive pole: Example 26 No occurrence (intermetallic com- 3.1 μm) pound layer at positive pole: Example 27 No occurrence (intermetallic com- 2.8 μm) pound layer at positive pole: Example 28 No occurrence (intermetallic com- 3.0 μm) pound layer at positive pole: Example 29 No occurrence (intermetallic com- 2.5 μm) pound layer at positive pole: Example 30 No occurrence (intermetallic com- 2.7 μm) pound layer at positive pole: Example 31 No occurrence (intermetallic com- 2.3 μm) pound layer at positive pole: Example 32 No occurrence (intermetallic com- 2.4 μm) pound layer at positive pole: Example 33 No occurrence (intermetallic com- 2.9 μm) pound layer at positive pole: Example 34 No occurrence (intermetallic com- 2.4 μm) pound layer at positive pole: Example 35 No occurrence (intermetallic com- 3.0 μm) pound layer at positive pole: Example 36 No occurrence (intermetallic com- 2.4 μm) pound layer at positive pole: Example 37 No occurrence (intermetallic com- 3.0 μm) pound layer at positive pole: Example 38 No occurrence (intermetallic com- 2.7 μm) pound layer at positive pole: Example 39 No occurrence (intermetallic com- 2.8 μm) pound layer at positive pole: Example 40 No occurrence (intermetallic com- 2.4 μm) pound layer at positive pole: Example 41 No occurrence (intermetallic com- 3.0 μm) pound layer at positive pole: Example 42 No occurrence (intermetallic com- 3.0 μm) pound layer at positive pole: Example 43 No occurrence (intermetallic com- 3.1 μm) pound layer at positive pole: Example 44 No occurrence (intermetallic com- 2.5 μm) pound layer at positive pole: Example 45 No occurrence (intermetallic com- 2.7 μm) pound layer at positive pole: Example 46 No occurrence (intermetallic com- 2.3 μm) pound layer at positive pole: Example 47 No occurrence (intermetallic com- 2.6 μm) pound layer at positive pole: Example 48 No occurrence (intermetallic com- 2.5 μm) pound layer at positive pole: Example 49 No occurrence (intermetallic com- 2.5 μm) pound layer at positive pole: Example 50 No occurrence (intermetallic com- 3.2 μm) pound layer at positive pole: Example 51 No occurrence (intermetallic com- 2.6 μm) pound layer at positive pole: Example 52 No occurrence (intermetallic com- 3.1 μm) pound layer at positive pole: Example 53 No occurrence (intermetallic com- 2.2 μm) pound layer at positive pole: Example 54 No occurrence (intermetallic com- 2.7 μm) pound layer at positive pole: Example 55 No occurrence (intermetallic com- 2.6 μm) pound layer at positive pole: Example 56 No occurrence (intermetallic com- 2.7 μm) pound layer at positive pole: Example 57 No occurrence (intermetallic com- 2.4 μm) pound layer at positive pole: Example 58 No occurrence (intermetallic com- 2.0 μm) pound layer at positive pole: Example 59 No occurrence (intermetallic com- 3.2 μm) pound layer at positive pole: Example 60 No occurrence (intermetallic com- 3.3 μm) pound layer at positive pole: Example 61 No occurrence (intermetallic com- 3.1 μm) pound layer at positive pole: Example 62 No occurrence (intermetallic com- 3.0 μm) pound layer at positive pole: Example 63 No occurrence (intermetallic com- 2.7 μm) pound layer at positive pole: Example 64 No occurrence (intermetallic com- 2.8 μm) pound layer at positive pole: Example 65 No occurrence (intermetallic com- 2.4 μm) pound layer at positive pole: Example 66 No occurrence (intermetallic com- 2.5 μm) pound layer at positive pole: Example 67 No occurrence (intermetallic com- 2.9 μm) pound layer at positive pole: Example 68 No occurrence (intermetallic com- 3.1 μm) pound layer at positive pole: Example 69 No occurrence (intermetallic com- 2.6 μm) pound layer at positive pole: Example 70 No occurrence (intermetallic com- 2.3 μm) pound layer at positive pole: Example 71 No occurrence (intermetallic com- 2.7 μm) pound layer at positive pole: Example 72 No occurrence (intermetallic com- 2.6 μm) pound layer at positive pole: Example 73 No occurrence (intermetallic com- 2.3 μm) pound layer at positive pole: Example 74 No occurrence (intermetallic com- 2.3 μm) pound layer at positive pole: Example 75 No occurrence (intermetallic com- 3.0 μm) pound layer at positive pole: Example 76 No occurrence (intermetallic com- 2.9 μm) pound layer at positive pole: Example 77 No occurrence (intermetallic com- 2.3 μm) pound layer at positive pole: Example 78 No occurrence (intermetallic com- 2.4 μm) pound layer at positive pole: Example 79 No occurrence (intermetallic com- 2.6 μm) pound layer at positive pole: Example 80 No occurrence (intermetallic com- 2.7 μm) pound layer at positive pole: Example 81 No occurrence (intermetallic com- 3.0 μm) pound layer at positive pole: Example 82 No occurrence (intermetallic com- 2.7 μm) pound layer at positive pole: Example 83 No occurrence (intermetallic com- 2.6 μm) pound layer at positive pole: Example 84 No occurrence (intermetallic com- 2.6 μm) pound layer at positive pole: Example 85 No occurrence (intermetallic com- 2.9 μm) pound layer at positive pole: Example 86 No occurrence (intermetallic com- 3.0 μm) pound layer at positive pole: Example 87 No occurrence (intermetallic com- 2.9 μm) pound layer at positive pole: Example 88 No occurrence (intermetallic com- 2.7 μm) pound layer at positive pole: Example 89 No occurrence (intermetallic com- 2.6 μm) pound layer at positive pole: Example 90 No occurrence (intermetallic com- 2.8 μm) pound layer at positive pole: Example 91 No occurrence (intermetallic com- 2.7 μm) pound layer at positive pole: Example 92 No occurrence (intermetallic com- 2.6 μm) pound layer at positive pole: Comparative Occurrence (intermetallic com- 100 μm) Example 1 pound layer at positive pole: Comparative Occurrence (intermetallic com- 80 μm) Example 2 pound layer at positive pole: Comparative Occurrence (intermetallic com- 110 μm) Example 3 pound layer at positive pole: Comparative Occurrence (intermetallic com- 70 μm) Example 4 pound layer at positive pole: Comparative Occurrence (intermetallic com- 90 μm) Example 5 pound layer at positive pole: Comparative Occurrence (intermetallic com- 85 μm) Example 6 pound layer at positive pole: Comparative Occurrence (intermetallic com- 90 μm) Example 7 pound layer at positive pole: Comparative Occurrence (intermetallic com- 50 μm) Example 8 pound layer at positive pole: Comparative Occurrence (intermetallic com- 85 μm) Example 9 pound layer at positive pole:

Pb-free soldering alloy powder which includes either Cu or In alone in addition to Sn was prepared. Then the soldering alloy powder of 90 wt. % and a pasty flux of 10 wt. % were mixed to form a solder paste. Using the solder paste the test chip and the circuit board were soldered. Electro-migration tests at respective solder bonding portions found out occurrences of electro-migration there. In that solder bonding portion, an unusually thick inter-metallic compound layer (50˜100 micro meter in thickness) was formed on the positive pole side which cannot be found in normal solder bonding portions. Meanwhile on the negative pole side a phenomenon of a partial loss of Copper wiring on the circuit board was confirmed. These phenomena may be deduced that due to very high current density (estimated value of 20 kA/cm²) on the solder bonding portion, a diffusion of atoms which normally never occurs has occurred and metallic atoms moved toward the positive pole to which electric current flows. Based on this we concluded that the electro-migration has occurred at the solder bonding portion.

On the other hand a Pb-free soldering alloy powder which includes both Cu of 0.1˜3.0 wt. % and In of 0.5˜8.0 wt. % simultaneously in addition to Sn was prepared. Then the soldering alloy powder of 90 wt. % and pasty flux of 10 wt. % were mixed to form a solder paste. Using the solder paste the test chip and the circuit board were soldered. Electro-migration tests at solder bonding portions found out that the thickness of an inter-metallic compound layer at respective solder bonding portions was less than 10 micro meters and that the occurrence of electro-migration was not confirmed.

Further, a Pb-free soldering alloy powder which includes both Cu of 0.1˜3.0 wt. % and In of 0.5˜8.0 wt. % simultaneously in addition to Sn and also includes either one of 1) Ag, Sb, Ni, Co, Fe, Mn, Cr as a strength improvement element, 2) Bi, Zn as a melting point drop element and 3) P, Ga and Ge as an anti-oxidant element was prepared respectively. Then the respective soldering alloy powder of 90 wt. % and pasty flux of 10 wt. % were mixed to form a respective solder paste. Using the respective solder paste a test chip and a circuit board were soldered. Electro-migration tests on solder bonding portions found out that a thickness of an inter-metallic compound layer at respective solder bonding portions using the respective solder pastes was less than 10 micro meters and that the occurrence of electro-migration was not confirmed.

Based on above experimental results it is found out that the soldering paste for soldering micro components of present invention which is a mixture of a powdered Pb-free soldering alloy comprising Cu of 0.1˜3.0 wt. % and In of 0.5˜8.0 wt. % simultaneously in addition to Sn; and a pasty flux; effectively suppresses electro-migration occurring at a solder bonding portion, and thereby provides a reliable and high quality Pb-free solder bonding portion even when an area of the solder bonding portion will become smaller due to progressive refinement of a mount board.

The present invention is applicable not only to a reflow soldering method but also to various kinds of soldering methods such as a flow soldering method, a dip soldering method, a laser soldering method, an ultrasonic soldering method, a friction soldering method and a manual soldering method using a resin flux cored solder, where an occurrence of electro-migration is anticipated due to a small area of a solder bonding portion. 

What is claimed is:
 1. A solder paste for bonding micro components comprising: a powdered Pb-free soldering alloy comprising Cu of 0.1˜3.0 wt. %, In of 0.5˜8.0 wt. %, Sn and unavoidable impurities of the rest; and a pasty or liquid flux; the powdered Pb-free soldering alloy and the pasty or liquid flux being mixed; whereby effectively suppressing electro-migration occurring at a solder bonding portion.
 2. The solder paste for bonding micro components of claim 1, the powdered Pb-free soldering alloy further comprising Ag and/or Sb of equal to or less than 4.0 wt. % as a strength improvement element, whereby effectively suppressing the electro-migration occurring at the solder bonding portion.
 3. The solder paste for bonding micro components of claim 1, the powdered Pb-free soldering alloy further comprising one or more elements selected from a group of Ni, Co, Fe, Mn and Cr of equal to or less than 0.3 wt. % as a strength improvement element, whereby effectively suppressing the electro-migration occurring at the solder bonding portion.
 4. The solder paste for bonding micro components of claim 1, the powdered Pb-free soldering alloy further comprising Bi and/or Zn of equal to or less than 3.0 wt. % as a melting point drop element, whereby effectively suppressing the electro-migration occurring at the solder bonding portion.
 5. The solder paste for bonding micro components of claim 1, the powdered Pb-free soldering alloy further comprising one or more elements selected from a group of P, Ga and Ge of equal to or less than 0.3 wt. % as an anti-oxidant element, whereby effectively suppressing the electro-migration occurring at the solder bonding portion.
 6. The solder paste for bonding micro components of claim 2, the powdered Pb-free soldering alloy further comprising one or more elements selected from a group of Ni, Co, Fe, Mn and Cr of equal to or less than 0.3 wt. % as a strength improvement element, whereby effectively suppressing the electro-migration occurring at the solder bonding portion.
 7. The solder paste for bonding micro components of claim 2, the powdered Pb-free soldering alloy further comprising Bi and/or Zn of equal to or less than 3.0 wt. % as a melting point drop element, whereby effectively suppressing the electro-migration occurring at the solder bonding portion.
 8. The solder paste for bonding micro components of claim 2, the powdered Pb-free soldering alloy further comprising one or more elements selected from a group of P, Ga and Ge of equal to or less than 0.3 wt. % as an anti-oxidant element, whereby effectively suppressing the electro-migration occurring at the solder bonding portion. 